This invention relates to sheet material, especially a base sheet obscured by an opaque but transparentizable microporous, diffusely light-reflective layer.
For centuries paper has been one of the most versatile substances made by man. Formed from commonly available cellulosic materials, it can be made stiff or flexible, rough or smooth, thick or thin, and provided with any desired color. After it has served its intended purpose, it can often be repulped and used again. In recent years, however, the demands for paper have increased to the extent that it has finally been recognized that the sources of cellulosic raw materials are not inexhaustible. Further, the energy required to manufacture paper is a significant consideration in a world becoming increasingly aware that supplies of energy are also finite. It has also become recognized that, where paper is used as a carrier for indicia, it can generally be used only once, it being impossible or impractical to remove indicia which are no longer needed or desired. There has thus arisen a desire for a substitute for conventional paper, especially one which can be repeatedly and easily reused; even a substitute which was more expensive to manufacture would be less expensive in the long run if it could be reused a sufficient number of times.
Several U.S. patents (e.g., U.S. Pat. Nos. 2,299,991, 3,031,328 and 3,508,344) disclose composite sheet material in which a light-colored opaque blushed lacquer layer is coated over a base sheet that is either dark-colored or imprinted with dark-colored indicia. The opacity and light color of the blushed lacquer coating are due to the inclusion of numerous microvoids; the local application of (1) heat or pressure (either of which irreversibly collapses the microvoids) or (2) a non-solvent liquid having substantially the same refractive index as the lacquer (which fills the microvoids), causes the coating to become selectively transparent and the underlying dark backing to become visible. A liquid employed to impart transparency to the opaque microporous layer can subsequently be evaporated to restore the original appearance.
U.S. Pat. No. 2,854,350 describes structures which are functionally similar to those just described, except that the blushed lacquer coatings are replaced by a microporous layer of finely divided calcium carbonate in an organic binder. Transparency is imparted by locally applying pressure or treating selected areas with a wax, oil or grease having a refractive index similar to that of the calcium carbonate; see U.S. Pat. No. 2,854,350. Other pigments may be incorporated in a microporous highly plasticized resin binder; see U.S. Pat. No. 3,247,006.
It is sometimes desirable to have microvoid-containing sheet material which can be transparentized by applying a liquid, but which cannot readily be transparentized by the application of heat or pressure. In such circumstances, a microvoid-containing layer of the type described in U.S. Pat. No. 4,299,880, owned by applicant's assignee, is preferred. This patent discloses a structure in which the microvoid-containing layer consists essentially of particles held in pseudo-sintered juxtaposition by a thermoset binder and has a cohesion value of at least 400 grams force*. FNT The cohesion value is determined by knife-coating a dispersion of a putative composition on a cleaned gray cold rolled steel panel, drying and curing as appropriate for the composition, to provide a coating 50-60 micrometers thick. Using a "Balance Beam Scrape-Adhesion and Mar Tester", sold by Gardner Laboratories, Inc., Bethesda, Md., a sapphire-tipped stylus is lowered into contact with the test panel and held in fixed position while a ball bearing-supported platform moves the panel. The minimum grams-force required to form a 50-micrometer deep scratch in the coating in a single pass is determined at a magnification of 40X and reported as cohesion value.
One important use for the microvoid-containing sheet materials just described is in the manufacture of re-usable order blanks for the fast food industry. Order blanks of this type are typically printed with a list of the various products offered for sale, as well as the number of each type of item which a customer might request. Upon receiving an order, the serving person utilizes a pen filled with a volatile clear "ink" to apply markings in the appropriate places; wherever the "ink" is applied, the microvoid layer is locally transparentized, allowing the dark backing to be seen. An order blank marked in this manner can then be fed into an optical mark recognition device, which automatically computes the total cost of the merchandise purchased. After a time, the volatile "ink" evaporates, so that the order blank can be used again.
One problem encountered with order blanks of the type just described is inadvertent contamination by various liquids in the area. For example, if cooking oil contacts the face of the order blank, it penetrates the microvoids and imparts local transparency. Since such oil is essentially non-volatile, a permanent mark may remain, causing erroneous readings when the order blank is scanned by the optical mark, recognition device. To a lesser extent, similar problems may be caused by spilling soft drinks, coffee, syrup or other water-based liquids on the surface of the order blank. Even if an oil- or water-repellent treatment is applied to the surface of the microvoid layer, the treatment may retard but not completely prevent penetration of the contaminating liquid, especially over an extended period of time. When used blanks are subsequently stacked on top of each other until the volatile "ink" has evaporated, the weight of the stack forces contaminating liquids into the microvoid layer, causing random marks and future erroneous readings by the optical scanner.